1. Field of the Invention
The present invention relates to a solid state laser apparatus generating a high quality and high power laser beam, and to a laser machining apparatus performing laser machining by using the high quality laser beam generated from the laser apparatus.
2. Description of the Prior Art
FIGS. 1(a) and (b) are sectional views showing a conventional laser apparatus disclosed in, for example, U.S. Pat. No. 3,803,509. In FIGS. 1(a) and (b), reference numeral 1 means a total reflection mirror, 2 means a partial reflection mirror, and 3 is a solid state component including an active solid state medium. In YAG laser as an example, Nd is doped as the active solid state medium is doped to produce Nd:YAG (Yttrium Aluminum Garnet), and the Nd:YAG has a surface roughness of about 50 .mu.inch RMS in order to avoid parasitic oscillation. Further, reference numeral 4 means a light source such as an arc lamp, 5 means a power source to turn ON the light source, and 6 is a condenser for the light source. For example, the condenser 6 has an elliptical sections and an inner surface thereof includes a light reflecting surface. Reference numeral 14 means a laser beam generated in a laser resonator including the mirrors 1 and 2, and 9, 900 mean cylindrical pipes adjusting a flow of a medium 70 which peripherally cools the light source 4 and the solid state component 3 including the active medium. The cylindrical pipe 900 has a rough surface. Reference numeral 7 means a seal material such as an O ring, 81, 82 mean an inflow opening and an outflow opening for the cooling medium 70, 15 is a laser beam externally derived, and 8 is a base.
The conventional solid state laser apparatus is provided as set forth above. In the apparatus, the light source 4 and the solid state medium 3 are disposed on a focal point of the condenser having the elliptical section. The light source 4 is turned ON by the power source 5 to emit light, and the emitted light is diffused at a stage to pass through the cylindrical pipe 900 having the rough surface so as to be peripherally incident on the solid state medium uniformly. The solid state medium is excited by the light to become a laser medium. Spontaneous emission light is generated by the laser medium, and is amplified during reciprocation in the resonator including the mirrors 1 and 2. When the spontaneous emission light has a predetermined magnitude or more, the light is externally emitted as the laser beam 15 having good directivity. In addition, the light source 5 and the solid state component 3 are peripherally cooled by the cooling medium 70 circulating in the cylindrical pipe 900.
In the conventional laser apparatus as set forth above, the light emitted from the light source peripherally irradiates and excites the solid state component uniformly. However, the solid state component is strongly excited in a vicinity of an intermediate portion thereof. Accordingly, an excitation distribution occurs so that variations are generated in quality of the laser medium generated in the section. As a result, it is impossible to provide a high quality beam having a good condensing performance.
FIG. 2 shows the results of an experiment in which the spontaneous emission light was observed from an axial direction of the solid state component without an optical resonator. The experiment was performed according to U.S. Pat. No. 3,803,509, and the experimental results are disclosed in "Applied Optics" vol. 14, No. 5, pp. 1192.
In FIG. 2, intensity of the spontaneous emission light is illustrated by the contour lines, and light intensity is the highest at an intermediate portion and the light intensity decreases toward a peripheral portion in a sectional direction.
Since light from the light source can peripherally irradiate the solid state component uniformly because of the cylindrical pipe 900 having the rough surface. there is a uniform distribution in a peripheral direction. However, it can be also seen that an extremely concentrated distribution is generated, in particular, at the peripheral portion in the sectional direction.
Further, in the conventional solid state laser apparatus, it is possible to use only light having a specified wavelength for laser output in the light (energy) from the exciting light source 4 absorbed by the solid state component 3. The light having other wavelengths is absorbed by the solid state component 3 so as to heat the solid state component 3. Hence, though it is necessary to cool the solid state component 3, there is no choice but to peripherally cool the solid state component 3 due to structural limitation of the solid state laser apparatus. As a result, a temperature distribution occurs in the section of the solid state component 3, and has the hot intermediate portion thereof and the peripheral portion which is colder than the intermediate portion.
Consequently, refractive index of the solid state component 3 has a distribution corresponding to the temperature distribution in the section, resulting in a distorted wave front of the laser light 9 passing through the solid state component 3. Hence, the laser light 9 in the resonator can be typically condensed. Such a laser beam condensing action by the solid state component is referred to as heat lens action of a solid state component. The heat lens action of the solid state component causes an unstable operation of the laser resonator. As a result, a sectional area of the laser light 9 in the laser resonator decreases, and the emitted laser beam 15 would be irregularly diverged since a magnitude of the heat lens action is varied according to variation in making power supply of the power source 5. Accordingly, there are several problems in that, for example, the laser beam deviates from a transmission mirror when the laser beam is transmitted by the transmission mirror into a machining stage or the like using the laser beam, and stable machining can not be performed because a focus position of the laser beam can be accurately defined until the heat lens action is stabilized when the laser beam is condensed by a lens so as to be used for machining.